Calculations in the dark room

Is there an electronic device, like the old HP Calculator, a palm pilot,
etc., and formulae which I can use in the dark room? For example, if I
determine that the exposure I want at 58cm is 12 sec at f/8, but then I
change the distance to 68 cm, how would I calculate the new exposure? (I
know that light intensity falls off as the square of the distance) And, if I
first decide to use Magenta=80 (on a Saunders LPL 45) but then go to
Magenta=120 Y=9, besides trial and error, are there formulae for
recalculating the exposure and is there a preferred device to make these
calculations on?

Thanks.

RON
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B&W Landscape and Still Photography: http://www.ronaldgansphotography.com